Update CONTRIBUTING.md

Author: etpinard

CONTRIBUTING.md

@@ -20,7 +20,7 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 
 | Label | Purpose |
 |--------|---------|
-| bug | bug reports confirmed by a plotly team member |
+| bug | bug report confirmed by a plotly team member |
 | enhancement | planned feature or task |
 | community | issue left open for community input and pull requests |
 | duplicate | *self-explanatory* |